Wireless communication system, wireless communication device and wireless communication method, and computer program

ABSTRACT

A radio communication network that requires no connection between base stations, for example, during handoff or when the cell is overloaded, is realized by using a receiver that has a multi-user detection function and a transmitter for synthesizing control information and user data to output the synthesized data. Upon reception of a signal that is transmitted from a mobile station located on the boundary with a neighboring cell, a base station synthesizes user data for a mobile station in its local cell and control information for controlling handoff operation to transmit the synthesized data. In response to this transmission, a mobile station in the neighboring cell separates the control information by multi-user detection to detect the signal and transmits a handoff request to the base station.

TECHNICAL FIELD

[0001] The present invention relates to radio communication systems inwhich a plurality of mobile terminals simultaneously communicates withone base station by multiple-access, radio communication apparatuses andradio communication methods, and computer programs and, moreparticularly, to a radio communication system having a multi-cellmultiple-access structure in which the interference within and outside acell is eliminated to expand the capacity (communication capability), aradio communication apparatus and radio communication method, and acomputer program.

[0002] More specifically, the present invention relates to radiocommunication systems in which repetition of frequency at shorterintervals increases the communication capability, radio communicationapparatuses and radio communication methods, and computer programs and,more particularly, to a radio communication system that requires noconnection between base stations, for example, during handoff or whenthe cell is overloaded, a radio communication apparatus and radiocommunication method, and a computer program.

BACKGROUND ART

[0003] Mobile communication that has originated from the discovery ofradio waves has been under research and development because of thenecessity of communication with vessels, aircrafts, and trains. Thetargets of communication have been expanded into automobiles and humanbeings. Not only telegraphic and telephone messages but also computerdata and multimedia content such as images can be transmitted throughmobile communication.

[0004] In recent years, reduction in size and cost of mobile terminalshas rapidly advanced owing to the improvement of manufacturingtechnology and the like. Expansion of information communication servicesand so on causes mobile terminals to be personalized, like mobilephones. Additionally, deregulation of communication and reduction ofcommunication fee causes increased growth in user population.

[0005] Basically, in mobile communication, a mobile station, such as anin-vehicle phone and a mobile phone, detects the nearest base station tocommunicate radio waves between the mobile station and the base station.A range over which the radio waves sent from one base station canpropagate is referred to as a “cell”. The cell is ordinarily a circlehaving a predetermined radius around an antenna mounted at the basestation. Arrangement of cells with no space therebetween constitutes acommunication service area.

[0006]FIG. 15 schematically illustrates the cell structure in a mobileradio communication system, typified by a cellular system, in which theservice area is two-dimensionally expanded with a plurality of basestations. Providing base stations (not shown) at predetermined intervalsand continuously (non-sparsely) arranging a plurality of cells offeredby the respective base stations, as shown in FIG. 15, allow a mobilestation to communicate with the base stations from anywhere, thusconstituting a wide service area.

[0007] The advantages of the cells that are used in the mobilecommunication system include the following ones; that is, thepropagation of the radio waves that are sent from a base station onlywithin the corresponding cell causes the same frequency to be repeatedlyused in other cells for efficiently utilizing limited frequencyresources, and the division into the cells causes the radio-wave outputfor communication to be decreased for reducing in size of a mobile bodythat is usually installed as a battery-driven mobile device and forsaving electric power. Recently, the reduction in size of the cells isincreasingly advanced owing to the increase of the number of mobilephone users and so on.

[0008] A plurality of mobile terminals exists in one cell and theysimultaneously communicate with one base station. In other words, fromthe point of the base station, it is necessary to detect which signal istransmitted from each user by multiple-access, that is, by multiplexingradio signals (multi-user detection).

[0009] Multiple-access technologies in radio communication include timedivision multiple access (TDMA) and frequency division multiple access(FDMA).

[0010] The TDMA is a communication system in which the communicationchannel is divided in advance into time slots on a time basis anddifferent time slots are allocated to the respective mobile terminals,which simultaneously communicate with each other. The TDMA is assumed tobe a digital system.

[0011] The FDMA is a communication system in which different frequenciesare allocated to the respective mobile terminals, which simultaneouslycommunicate with each other, (that is, different frequencies areallocated for every communication channel), to establish communication.Namely, multiple channels that are used for communication are arrangedon a frequency basis and idle channels are appropriately allocated tothe respective mobile terminals for use. The FDMA is accommodated toeither analog or digital communication system.

[0012] For example, in the same cell, a base station can simultaneouslyconnect to a plurality of mobile stations in its local cell by the TDMA.In such a case, one TDMA frame is divided into a plurality of timedomains, each being allocated to an uplink or a downlink of each mobilestation. A method, for example, channel allocation is employed for theallocation of the time domains.

[0013] Between cells, switching a frequency channel that is used toanother frequency channel for communication by the FDMA allows theproblems of interference waves from outside the local cell around thecell boundary to be eliminated.

[0014] Within one cell, the base station transmits (broadcasts) a beacon(identification signal) or other control information to performsynchronization within the local cell, identify the cell, specify thefrequency channel to be used, and so on.

[0015] How to enhance the communication capability with fewer resourcesposes a big task in a radio communication environment in which themobile communication has become quickly widespread and multiple mobilestations exist in one cell. In such a radio communication environment,the multi-user detection for detecting a plurality of users in a cell ishighly important. With a known radio communication system, there is nobenefit given by multiplexing signals for someone other than a desiredperson for transmission. This is because the interference is simplyincreased to lower the communication quality. In other words, there isno known radio communication system on the assumption of the multi-userdetection and of the synthesis of user data and the control informationto transmit the synthesized data.

[0016] Meanwhile, in the service area in which small cells are arrangedwith no space therebetween as shown in FIG. 15, the base station to beconnected must be shifted in connection with a mobile station that movesfrom one cell to another cell. Such a shift of the base station to beconnected at a mobile station is referred to as “handover” or “handoff”.The opportunity for the handoff is increased as the cells are downsized.

[0017] The mobile station must determine the base station to be linkedto next, that is, must perform cell detection, for every handoff. Forexample, the move of the mobile station toward the cell boundary causesthe communication quality of desired waves from the local base stationto be lowered and permits the reception of interference signals fromoutside the local cell, so that it is necessary for detecting anotherbase station having higher communication quality. The cell detection isperformed also on startup of the device.

[0018] In a known cellular system, the switching of the controlinformation between base stations through a control-switching stationstarts the handoff procedure. FIG. 16 is a diagram showing an example ofa multi-cell environment in a cellular communication system. Referringto FIG. 16, a mobile station 201 moves toward a cell 13 whilecommunicating with a base station 102 in a cell 12. The function forswitching the target of the communication signal for a subscriberstation from the transceiver in the original base station 102 to thetransceiver in a next transceiver 103, when the mobile station 201 is tocross the cell boundary, is referred to as “handoff”.

[0019] The handoff techniques in the cellular communication system aremainly divided into two categories; “soft handoff” and “hard handoff”.With either handoff technique, the communication must be establishedbetween the base stations for informing that the mobile station is atarget for the handoff when the mobile station switches the base stationto be connected in the cell boundary. Such communication between basestations is established through a backbone. Namely, an infrastructure,such as a mobile communication control center, for controlling the basestations is necessary, as shown in FIG. 17.

[0020] The hard handoff ordinarily occurs in the vicinity of the cellboundary. In this case, the base station continuously measures the powerof the received signal from the mobile station that is communicated withand determines whether the power of the received signal from the mobilestation in the vicinity of the cell boundary is lower than a ratedvalue. The base station in the candidate cell to be switched to and thebase station in the original cell must communicate the informationconcerning the handoff with each other through the backbone in order toquickly perform the handoff without suspending the call in progress.

[0021] The soft handoff is continuously performed within a range apartfrom the base station in the original cell and the base station in thecell to be switched to by a predetermined distance. In the soft handoff,when the mobile station moves in the vicinity of the boundary betweencells, it is connected to both base stations in the respective cells. Itis determined whether the switching is performed based on the receptionof a pilot signal from the mobile station. A telephone switching-center,that is, the control-switching station that exists on the backbone ofthe corresponding network determines the point where the switching fromthe base station in the original cell is performed.

[0022] With either handoff technique, it is essential to establish somekind of connection between the base stations in order to realize thecellular handoff in a multi-cell environment. Hence, the number ofcontrol-switching stations is increased in accordance with the increasein the number of the base stations, thus causing a problem in that thecellular network is complicated. Since the shift to higher frequencyband has been advanced in accordance with the request for high capacityand the request for high communication speed in recent years, it isanticipated, based on the propagation characteristic or the like, thatthe size of the cells is reduced, that is, the number of the basestations is increased. The reduction in traffic of the backbone thatconnects the base stations is an important concern.

[0023] When the number of users connected to a base station increases(that is, the cell is overloaded) and the number of users connected toanother base station next the above base station is small, a controlmethod is proposed for forcedly performing the handoff of the mobilestation that is connected to one base station while being located in thevicinity of the boundary between the base stations, to the other basestation. Also in such a case, the resource management information ateach base station must be communicated through the control-switchingstation by using the resources in the backbone, thus increasing thetraffic in the backbone.

[0024] In order to expand the calling areas with no space therebetweenby using a known handoff technique, the constructor of the base station(access point) must allocate the frequency according to planning suchthat the base station in a neighboring cell does not use the samefrequency for eliminating the interference. The infrastructure isdesigned so as to reduce the load of the mobile station by providing thebase station that is fixedly installed with various control functions.Under such design of a radio span, the mobile terminal is obliged torely on the base station as for the control, so that themobile-station-driven control operation cannot be expected in a radiolayer.

DISCLOSURE OF INVENTION

[0025] It is an object of the present invention to provide a superiorradio communication system in which a plurality of mobile terminalssimultaneously communicates with one base station by multiple-access, aradio communication apparatus and radio communication method, and acomputer program.

[0026] It is another object of the present invention to provide asuperior radio communication system in which a mobile station can switchthe base station to another base station outside its local cell withoutrequiring connection between the base stations, for example, duringhandoff or when the cell is overloaded, a radio communication apparatusand radio communication method, and a computer program.

[0027] It is another object of the present invention to provide asuperior radio communication system in which mobile-terminal-drivenhandoff can be realized even if the base stations are planlesslyprovided, a radio communication apparatus and radio communicationmethod, and a computer program.

[0028] The present invention provides, in its first aspect, a radiocommunication system having a multi-cell multi-user structure in whichcells where the respective base stations are capable of communicatingare adjacently or closely arranged. Each base station has a multi-userdetection function. The base station determines that a mobile stationoutside its local cell approaches the boundary with a neighboring cellbased on the detection of a transmitted signal from the mobile stationfor identifying a user, and mixes and synthesizes a signal for a mobilestation within the local cell and a signal for the mobile stationoutside the local cell to output the synthesized signal.

[0029] The “system” here refers to a logical group of a plurality ofdevices (or functional modules for realizing a certain function). Itdoes not matter whether each device or functional module is contained ina single casing (this is also true for the following description).

[0030] In the radio communication system according to the first aspectof the present invention, the base station can synthesize user data fora mobile station within the local cell and control information forcontrolling the handoff operation, which is for a mobile station outsidethe local cell, upon reception of a transmitted signal from a mobilestation located on the boundary with a neighboring cell, to transmit thesynthesized data. In response to this transmission, the mobile stationin the neighboring cell separates the control information by multi-userdetection to detect the signal and transmits a handoff request to thebase station. The base station returns a handoff-enabling signal when itaccepts the handoff request. The mobile station can disconnect from theoriginal base station to perform handoff. Accordingly, the amount oftraffic on a backbone does not need to be increased in the handover of amobile station because no connection between base stations is required.

[0031] The present invention provides, in its second aspect, a radiocommunication system having a multi-cell multi-user structure in whichcells where the respective base stations are capable of communicatingare adjacently or closely arranged. Each mobile station in themulti-user structure has a multi-user detection function. The mobilestation determines that the mobile station approaches the boundary witha neighboring cell based on the detection of a transmitted signal from abase station outside its local cell for identifying a user, and mixesand synthesizes a signal for the base station within the local cell anda signal for the base station in the neighboring cell to output thesynthesized signal.

[0032] In the radio communication system according to the second aspectof the present invention, the mobile station can synthesize user datafor the base station within the local cell and control information forcontrolling the handoff operation, which is for the base station of theneighboring cell, upon reception of a transmitted signal from the basestation of a neighboring cell, to transmit the synthesized data. Inresponse to this transmission, the base station in the neighboring cellseparates the control information by multi-user detection to detect thesignal and returns a handoff-enabling signal when it accepts a handoffrequest. The mobile station can disconnect from the original basestation to perform handoff. Accordingly, as in the first aspect of thepresent invention, the amount of traffic on the backbone does not needto be increased in the handover of a mobile station because noconnection between base stations is required.

[0033] The present invention provides, in its third aspect, a radiocommunication apparatus or a radio communication method operating in aradio communication environment that has a multi-cell multi-userstructure in which cells where the respective base stations are capableof communicating are adjacently or closely arranged. The radiocommunication apparatus or the radio communication method includesmulti-user detection means or a multi-user detection step for detectinga plurality of received signals as desired signals, and multiple-signaltransmission means or a multiple-signal transmission step for mixing andsynthesizing a signal for a station within a local cell and a signal fora station outside the local cell in response to the detection of atransmitted signal from the station outside the local cell foridentifying a user by the multi-user detection means or in themulti-user detection step to output the synthesized signal.

[0034] In the radio communication apparatus or method according to thethird aspect of the present invention, for example, user data for astation within a local cell is mixed with control information for astation outside the local cell to transmit the mixed data. At a receiverside, it is possible to demodulate the control information as desiredsignals by multi-user detection. Accordingly, the amount of traffic onthe backbone does not need to be increased in the handover of a mobilestation or when a cell is overloaded, even if no connection isestablished between base stations, because the control information thatis mixed with the ordinary user data can be transmitted and receivedbetween cells.

[0035] The multi-user detection means or step can be realized byapplying an interference cancellation technique such as so-called SIC(Successive Interference Cancellation). Specifically, the multi-userdetection means or step detects all the received signals by demodulatingthe received signals, each being the sum of noise and an incoming signalthat is transmitted from each transmitting station within and outsidethe local cell and is propagated through each propagationcharacteristic, in the descending order of its received power, andrepeating a process of canceling the signals at a receiving station.

[0036] In order to preferably realize the multi-user detection inaccordance with the received power at a receiver side, themultiple-signal transmission means or step preferably exchanges thetransmission power of a signal for a station within the local cell forthat of a signal for a station outside the local cell to output them ata transmitter side.

[0037] When the radio communication apparatus or method according to thethird aspect of the present invention operates as a base station that isconnected to one or more mobile stations in its local cell, themultiple-signal transmission means or step may mix and synthesize asignal for a mobile station within the local cell and a signal for amobile station outside the local cell to output the synthesized signal,upon determining that the mobile station outside the local cellapproaches the boundary with a neighboring cell based on the detectionof a transmitted signal from the mobile station outside the local cellfor identifying the user by the multi-user detection means or in themulti-user detection step. The radio communication apparatus or methodmay enable handoff in response to a request for the handoff from themobile station outside the local cell.

[0038] When the radio communication apparatus or method according to thethird aspect of the present invention operates as a mobile station thatis located in a cell provided by a base station, the multiple-signaltransmission means or step may mix and synthesize a signal for the basestation within the local cell and a signal for the base station in aneighboring cell to output the synthesized signal, upon determining thatthe mobile station approaches the boundary with the neighboring cellbased on the detection of a transmitted signal from a base stationoutside the local cell for identifying the user by the multi-userdetection means or in the multi-user detection step. The radiocommunication apparatus or method may perform handoff in response tohandoff allowance from the base station in the neighboring cell.

[0039] The present invention provides, in its fourth aspect, a radiocommunication apparatus or a radio communication method operating in aradio communication environment having a multi-user structure. The radiocommunication apparatus or the radio communication method includessignal generation means or a signal generation step for generating aplurality of transmitted signals for a plurality of transmissiontargets, and multiplex-synthesis transmission means or amultiplex-synthesis transmission step for mixing and synthesizing thetransmitted signals and changing the transmission power of thetransmitted signals for every transmission target. when outputting them.

[0040] In the radio communication environment having a multi-userstructure, the radio communication apparatus operating as a receiverreceives the transmitted signals from the plural transmission sources asthe sum of signals that are supplied through the respective propagationcharacteristics. The individual signals can be demodulated as desiredsignals by performing the multi-user detection in accordance with thereceived power by applying an interference cancellation technique suchas SIC. With the radio communication apparatus or the radiocommunication method according to the fourth aspect of the presentinvention, the plural transmitted signals are mixed and synthesized tooutput the synthesized signal by changing the transmission power forevery transmission target on the assumption that the multi-userdetection in accordance with the received power is performed at areceiver side. Accordingly., each transmitted signal can be preferablydetected for identifying a user in accordance with the received power atthe receiver side.

[0041] The present invention provides, in its fifth aspect, a computerprogram that is described in a computer-readable form to cause acomputer system to execute a process for radio communication in a radiocommunication environment having a multi-cell multi-user structure-inwhich cells where the respective base stations are capable ofcommunicating are adjacently or closely arranged. The computer programincludes a multi-user detection step for detecting plural receivedsignals as desired signals and a multiple-signal transmission step formixing and synthesizing a signal for a station within a local cell and asignal for a station outside the local cell to output the synthesizedsignal in response to the detection of a transmitted signal from thestation outside the local cell for identifying a user in the multi-userdetection step.

[0042] The computer program according to the fifth aspect of the presentinvention defines a computer program that is described in acomputer-readable form, for realizing a certain process on a computersystem. In other words, the installation of the computer programaccording to the fifth aspect of the present invention on the computersystem causes the computer system to carry out a cooperative operation,thus achieving an operational effect similar to the one achieved in theradio communication apparatus or the radio communication methodaccording to the second aspect of the present invention.

[0043] The above and other objects, features, and advantages of thepresent invention will become clear from the following description ofthe preferred embodiments taken in conjunction with the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044]FIG. 1 is a diagram schematically showing the structure of amulti-cell multiple-access radio communication environment.

[0045]FIG. 2 is a diagram schematically showing the channel structure ofa mobile station MS-A in the reception.

[0046]FIG. 3 is a diagram schematically showing the functional structureof a receiver side receiving a received signal R_(A) that is the sum oftwo transmitted signals R_(XA) and R_(YA) and noise.

[0047]FIG. 4 is a diagram showing in detail the internal structure of amulti-user detection entity 31 that is structured by using SIC.

[0048]FIG. 5 is a diagram schematically showing a multi-cellmultiple-access radio communication environment, according to anembodiment of the present invention, in which handoff operation isperformed.

[0049]FIG. 6 is a diagram schematically showing the functional structurefor performing multi-user detection at a base station in the multi-cellmultiple-access radio communication environment shown in FIG. 5.

[0050]FIG. 7 is a diagram schematically showing the functional structurefor performing the multi-user detection at a mobile station in themulti-cell multiple-access radio communication environment shown in FIG.5.

[0051]FIG. 8 is a sequence diagram showing the operating procedure forperforming base-station-driven handoff of the mobile station in themulti-cell multiple-access radio communication system structureaccording to an embodiment of the present invention.

[0052]FIG. 9 is a flowchart showing the operating procedure of a radiocommunication apparatus operating as a mobile station in the multi-cellradio communication environment according to an embodiment of thepresent invention.

[0053]FIG. 10 is a flowchart showing the operating procedure of a radiocommunication apparatus operating as a base station in the multi-cellradio communication environment according to an embodiment of thepresent invention.

[0054]FIG. 11 is a sequence diagram showing the operating procedure forperforming mobile-station-driven handoff of the mobile station in themulti-cell multiple-access radio communication system structureaccording to an embodiment of the present invention.

[0055]FIG. 12 is a diagram showing a structure example of a packetsynthesized by TDMA.

[0056]FIG. 13 is a diagram showing an example in which the handoff isrealized without a mechanism for controlling plural access points.

[0057]FIG. 14 is a diagram showing an example in which the handoff isrealized in a ubiquitous network.

[0058]FIG. 15 is a diagram schematically showing the cell structure in amobile radio communication system in which a service area istwo-dimensionally expanded with plural base stations.

[0059]FIG. 16 is a diagram showing an example in which a mobile station201 moves toward a cell 13 while communicating with a base station 102in a cell 12.

[0060]FIG. 17 is a diagram exemplifying an infrastructure of a mobilecommunication control center that controls plural base stations.

BEST MODE FOR CARRYING OUT THE INVENTION

[0061] The present invention realizes a cellular network that requiresno connection between base stations, for example, during handoff or whenthe cell is overloaded, by using a receiver that has a multi-userdetection function and a transmitter for synthesizing controlinformation and user data to output the synthesized data.

[0062] Namely, upon reception of a signal that is transmitted from amobile station located on the boundary with a neighboring cell, a basestation synthesizes user data for a mobile station in its local cell andcontrol information for controlling handoff operation to transmit thesynthesized data. In response to this transmission, the mobile stationin the neighboring cell separates the control information by multi-userdetection to detect the signal and transmits a handoff request to thebase station. On the other hand, when the mobile station requestshandoff, it synthesizes user data and control information forcontrolling handoff operation to transmit the synthesized data. Inresponse to this transmission, the base station in a neighboring cellseparates the control information by multi-user detection to detect thesignal and receives the handoff request.

[0063] In order to perform the multi-user detection, that is, to detecta plurality of signals, an interference cancellation technique such asSIC (Successive Interference Cancellation) can be applied. With the SIC,repetition of the process of demodulating the received signals, eachbeing the sum of an incoming signal that has been propagated from eachtransmitting station to a receiving station through the correspondingpropagation characteristic and noise, in the descending order of theirreceived power and of canceling signals at the receiving station allowsall the received signals to be detected. For example, a multiple-accesscommunication system, in which the multi-user detection by the SIC byusing all the received signals as desired signals is used-to realize amulti-cell structure in which one frequency is repeated by using anon-diffusion mechanism, is disclosed in Japanese Patent Application No.2002-7959 that has been already assigned to the applicant.

[0064] The multi-user detection by using the SIC technique will now bedescribed.

[0065] A multi-cell multiple-access radio communication environment, asshown in FIG. 1, is proposed. It is assumed that one frequency isrepeated in the non-diffusion mechanism that does not use a diffusionseries for the multiple access (that is, that does not perform CDMA(code division multiple access)).

[0066] In the example in FIG. 1, one mobile terminal MS-A exists in acell X in which a base station BS-X can establish communication and onemobile terminal MS-B exists in a cell Y provided by a base station BS-Y.It is assumed that the cell X is arranged next to the cell Y and thatboth the base station BS-X of the cell X and the base station BS-Y ofthe cell Y, which is arranged next to the cell X or adjacent to the cellX, can use the same frequency f, to connect to the respective mobilestations at the same time.

[0067] Although only one mobile terminal is provided in each cell forsimplifying the description and avoiding a complicated drawing, multiplemobile terminals may actually exist in each cell. It is to be understoodthat many adjacent cells are arranged around the cells X and Y toconstitute an extended communication service area.

[0068] At a certain time in FIG. 1, the mobile station MS-A establishescommunication with the base station BS-X and the mobile station MS-Bestablishes communication with the base station BS-Y. The same frequencyis used for communication between the base station and the mobilestation in both pairs and the base station and the mobile station inboth pairs are connected to each other at the same time.

[0069] The signals that the mobile station MS-A receives at its downline is the sum of signals that are transmitted from the base stationBS-X in the local cell to the mobile station MS-A and received in themobile station MS-A with a propagation characteristic H_(XA) (desiredwaves) and signals that are transmitted from the base station BS-Youtside the local cell to the mobile station MS-B and received in themobile station MS-A with a propagation characteristic H_(YA)(interference waves). Similarly, the signals that the mobile stationMS-B receives at its down line is the sum of signals that aretransmitted from the base station BS-Y to the mobile station MS-B andreceived in the mobile station MS-B with a propagation characteristicH_(YB) (desired waves) and signals that are transmitted from the basestation BS-X outside the local cell to the mobile station MS-A andreceived in the mobile station MS-B with a propagation characteristicH_(XB) (interference waves).

[0070] The signals that the base station BS-X receives at its up line isthe sum of signals that are transmitted from the mobile station MS-A inthe local cell to the base station BS-X and received in the base stationBS-X with a propagation characteristic H_(AX) (desired waves) andsignals that are transmitted from the mobile station MS-B outside thelocal cell to the base station BS-Y and received in the base stationBS-X with a propagation characteristic H_(BX) (interference waves).Similarly, the signals that the base station BS-Y receives at its upline is the sum of signals that are transmitted from the mobile stationMS-B in the local cell to the base station BS-Y and received in the basestation BS-Y with a propagation characteristic H_(BY) (desired waves)and signals that are transmitted from the mobile station MS-A outsidethe local cell to the base station BS-X and received in the base stationBS-Y with a propagation characteristic H_(AY) (interference waves).

[0071] Ordinarily, in order to suppress such interference, the signaltransmission toward stations outside the local cell is suppressed. Incontrast, according to the present invention, the signal transmissiontoward stations outside the local cell is affirmatively performed at thetransmitter side and both the signals within the local cell and thesignals from stations outside the local cell are demodulated by themulti-user detection at the receiver side.

[0072] In other-words, at least one of the base station and the mobilestation provides the multi-user detection function by using the SIC andcan receive the desired waves both from within its local cell and fromoutside its local cell. At a radio transmission side that communicateswith such a base station or a mobile station, two or more kinds oftransmission data, for example, user data and control information, canbe synthesized for transmitting the synthesized data. Such operatingcharacteristics can be positively utilized for, for example, handoffbetween cells, which will be described below.

[0073]FIG. 2 is a diagram schematically showing the channel structure ofthe mobile station MS-A in the reception. As shown in FIG. 1, the mobilestation MS-A receives the signals from the base station BS-X in itslocal cell and the signals that are transmitted from the base stationBS-Y outside its local cell to the mobile station MS-B, in its downline. A case in which the mobile station MS-A performs the multi-userdetection will be exemplified.

[0074] The base station BS-X performs signal processing such asmodulation or up-conversion for transmission information I_(XA) (001) bya transmitter X 11 to convert the transmission information I_(XA) to atransmitted signal T_(XA) (002) for transmission. The base station BS-Yperforms signal processing such as modulation or up-conversion fortransmission information I_(YB) (011) by a transmitter Y 21 to convertthe transmission information I_(YB) to a transmitted signal T_(YB) (012)for transmission.

[0075] The transmitted signal T_(XA) (002) is propagated to the mobilestation BS-X through a propagation characteristic H_(XA) and thetransmitted signal T_(YB) (012) is propagated to the mobile station BS-Xthrough a propagation characteristic H_(YA). Thermal noise is added onthe way to the mobile station BS-X. As a result, a received signal R_(A)(020) received at the mobile station MS-A is given by the followingequation:

R_(A)=H_(XA)T_(XA)+H_(YA)T_(TB)+noise

[0076] Since a receiving side that receives the received signal R_(A)(the mobile station MS-A in this case) detects the informationconcerning two information sources, that is, the base stations BS-X andBS-Y, it performs interference cancellation on the assumption that thenumber of users are two.

[0077]FIG. 3 is a diagram schematically showing the functional structureof a receiver side receiving the received signal R_(A) that is the sumof two transmitted signals R_(XA) and R_(YA) and noise. As shown in FIG.3, the receiver side includes a multi-user detection entity (MUD) 31that detects a user from the received signal R_(A) (020) andhard-decision decoders 32 and 33 for estimating transmission informationI_(XA) and I_(YB), respectively, by hard-decision decoding the twosignals extracted from the received signals.

[0078] The multi-user detection entity 31 performs the multi-userdetection on the assumption that the number of users is two. Accordingto this embodiment, the multi-user detection is performed by the SICbased on the strength of the received signals. In the SIC, all thereceived signals are detected by repetition of the process ofdemodulating the received signals in the descending order of theirreceived power and of canceling the signals at the receiver side. It isgenerally assumed that a user who transmits a signal of higher strengthhas higher reliability.

[0079]FIG. 4 is a diagram showing in detail the internal structure ofthe multi-user detection entity 31 that is structured by using the SIC.As shown in FIG. 4, the multi-user detection entity 31 has, as one basicunit of a interference cancellation process, a filter (Demod A) 41 forcorrecting the received signal R_(A) based on the propagationcharacteristic H_(XA) and estimating the transmitted signal T_(XA) fromthe base station BS-X by demodulation and soft decision (or harddecision) decoding, an interference canceller (IC) 42 for subtractingthe received signal that is transmitted from the base station BS-X fromall the received signals based on the estimated result for thetransmitted signal T_(XA), a filter (Demod B) 43 for correcting theresult of the interference cancellation based on the propagationcharacteristic H_(XA) and estimating the transmitted signal T_(YB) fromthe base station BS-Y by demodulation and soft decision (or harddecision) decoding, and an interference canceller (IC) 44 forsubtracting the received signal that is transmitted from the basestation BS-Y from all the received signals based on the estimated resultfor the transmitted signal T_(YB). The number of times the basic unit isrepeated is determined in consideration of the precision of signalprocessing and the processing cost.

[0080] When R_(A) is received through the corresponding propagationpath, a filter 41-1 outputs a replica (021) of the transmitted signalT_(XA) from the base station BS-X to the corresponding mobile stationMS-A by using the estimated value of the propagation characteristicH_(XA).

[0081] The multiplication of the replica (021) of the transmitted signalT_(XA) from the base station BS-X to the corresponding mobile stationMS-A by the estimated value of the propagation characteristic H_(XA) bya multiplexer 45-1 generates a replica (022) of a transmission pathoutput from the base station BS-X to the mobile station MS-A.

[0082] The replica (022) of the transmission path output from the basestation BS-X to the mobile station MS-A is an interference component forthe mobile station MS-B. An interference canceller 42-1 subtracts thereplica (022) that is an interference component from the received signalR_(A) (020) and supplies the subtracted result (023) to a filter 43-1.The filter 43-1 outputs a replica (024) of the transmitted signal T_(YB)from the base station BS-Y to the corresponding mobile station MS-B byusing the estimated value of the propagation characteristic H_(YB).

[0083] The multiplication of the replica (024) of the transmitted signalT_(YB) from the base station BS-Y to the corresponding mobile stationMS-B by the estimated value of the propagation characteristic H_(YB) bya multiplexer 46-1 generates a replica (025) of a transmission pathoutput from the base station BS-Y to the mobile station MS-B.

[0084] The replica (025) of the transmission path output from the basestation BS-Y to the mobile station MS-B is an interference component forthe mobile station MS-A. An interference canceller 44-1 subtracts thereplica (025) that is an interference component from the subtractedresult (023) in the interference canceller 42-1.

[0085] The addition of the replica (022) of the transmission path outputfrom the base station BS-X to the mobile station MS-A and the subtractedresult (026) in the interference canceller 44-1 by an adder 47-2generates an input (027) that is supplied to the next-stageinterference-cancellation-process unit 40-2. Similar interferencecancellation process is repeated.

[0086] When the number of times the interference cancellation process isrepeated reaches a predetermined number N, an output (028) from a filter41-N in the mobile station MS-A is supplied to a decoder A 51 thatoutputs a replica (029) of the transmission information I_(XA) suppliedto the mobile station MS-A. In the same manner, an output (030) from afilter 43-N in the mobile station MS-B is supplied to a decoder B 52that outputs a replica (031) of the transmission information I_(YB)supplied to the mobile station MS-B.

[0087] After the multi-user detection described above is completed,replicas (040) and (042) from the respective base stations BS-X and BS-Yundergo hard-decision decoding to generate decided values (041) and(043).

[0088] The mobile station MS-A utilizes the decided value (041) forI_(XA) among the decided values (041) and (043). Similarly, the mobilestation MS-B utilizes the decided value (043) for I_(YB) among thedecided values (041) and (043).

[0089] Consequently, although the base stations BS-X and BS-Y use thesame frequency f₁, they can be connected to the mobile stations MS-A andMS-B, respectively, at the same time. In other words, since therepetition by using one frequency can be realized, the communicationcapability is greatly increased. The non-diffusion mechanism, which doesnot use the diffusion series, uses a lower frequency band and thereforethe increase in communication capability can be anticipated.

[0090] Although the multi-user detection is described above in thecontext of the down line from the base station to the mobile station,the multi-user detection can be achieved even in the up line from themobile station to the base station if each base station implements theinterference cancellation technique such as the SIC.

[0091] According to this embodiment, the operating characteristics inwhich the receiver side has the multi-user detection function to receivethe desired waves from both within and outside its local cellis-affirmatively utilized for the handoff between cells and the like. Aradio communication environment shown in FIG. 5 will now be described.

[0092] In the radio communication environment in FIG. 5, which is amulti-cell structure in which one frequency is repeated by using anon-diffusion mechanism, multiple access is realized in each cell bytime division multiplexing. For simplicity, one mobile station 208 isoperating in a cell 12 that is structured by a base station 102 and onemobile station 209 is operating in a neighboring cell 13 that isstructured by a base station 103.

[0093]FIG. 6 is a diagram schematically showing the functional structurefor performing the multi-user detection at a base station in themulti-cell multiple-access radio communication environment shown in FIG.5.

[0094] The mobile station 208 includes a signal generator 381 forgenerating a transmitted signal 082 that has user data for the basestation 102 in its local cell, a signal generator 382 for generating atransmitted signal 083 that has control information for the base station103 outside its local cell, a multiplexer 383 for mixing the twotransmitted signals by time division multiplexing or the like, and aradio transmitter 384 for transmitting a multiplexed signal 001 as atransmitted signal 002. The entities of the signal generators 381 and382 are, for example, processing processes generated for everytransmission target. While the signal generator 382 ordinarily operateswhen the mobile station 208 is within the cell 12, the signal generator383 starts to operate when the mobile station 208 is located in thevicinity of the boundary with the neighboring cell 13 (described below).When the base stations 102 and 103 perform the multi-user detectionbased on the strength of the received signals, like the SIC, the radiotransmitter 384 preferably exchanges the transmission power for the userdata for that for the control information to output them.

[0095] The mobile station 209 includes a signal generator 391 forgenerating a transmitted signal 093 that has user data for the basestation 103 in its local cell, a signal generator 392 for generating atransmitted signal 092 that has control information for the base station102 outside its local cell, a multiplexer 393 for mixing the twotransmitted signals by time division multiplexing or the like, and aradio transmitter 394 for transmitting a multiplexed signal 005 as atransmitted signal 006. The entities of the signal generators 391 and392 are, for example, processing processes generated for everytransmission target. While the signal generator 391 ordinarily operateswhen the mobile station 209 is within the cell 13, the signal generator392 starts to operate when the mobile station 209 is located in thevicinity of the boundary with the neighboring cell 12 (described below).When the base stations 102 and 103 perform the multi-user detectionbased on the strength of the received signals, like the SIC, the radiotransmitter 394 preferably exchanges the transmission power for the userfor that for the control information to output them.

[0096] The base station 102 has a multi-user detector 3002. Thetransmitted signal 002 from the mobile station 208 is supplied as areceived signal 003 to the base station 102 through a propagation pathhaving a propagation characteristic H82, and the transmitted signal 006from the mobile station 209 is also supplied as a received signal 007 tothe base station 102 through a propagation path having a propagationcharacteristic H92. The multi-user detector 3002 detects the receivedsignal 003 for identifying the user by the multi-user detection functionsuch as the SIC to reproduce a signal 082′ as the signal generated bythe signal generator 381 and to reproduce a signal 083′ as the signalgenerated by the signal generator 382. Similarly, the multi-userdetector 3002 detects the received signal 007 for identifying the userto reproduce a signal 093′ as the signal generated by the signalgenerator 391 and to reproduce a signal 092′ as the signal generated bythe signal generator 392.

[0097] The base station 103 has a multi-user detector 3003. Thetransmitted signal 002 from the mobile station 208 is supplied as areceived signal 004 to the base station 103 through a propagation pathhaving a propagation characteristic H83, and the transmitted signal 006from the mobile station 209 is also supplied as a received signal 008 tothe base station 103 through a propagation path having a propagationcharacteristic H93. The multi-user detector 3003 detects the receivedsignal 004 for identifying the user by the multi-user detection functionsuch as the SIC to reproduce a signal 082″ as the signal generated bythe signal generator 381 and to reproduce a signal 083″ as the signalgenerated by the signal generator 382. Similarly, the multi-userdetector 3003 detects the received signal 008 for identifying the userto reproduce a signal 093″ as the signal generated by the signalgenerator 391 and to reproduce a signal 092″ as the signal generated bythe signal generator 392.

[0098]FIG. 7 is a diagram schematically showing the functional structurefor performing the multi-user detection at a mobile station in themulti-cell multiple-access radio communication environment shown in FIG.5.

[0099] The base station 102 includes a signal generator 421 forgenerating a transmitted signal 028 that has user data for the mobilestation 208 in its local cell, a signal generator 422 for generating atransmitted signal 029 that has control information for the mobilestation 209 outside its local cell, a multiplexer 423 for mixing the twotransmitted signals by time division multiplexing or the like, and aradio transmitter 424 for transmitting a multiplexed signal 009 as atransmitted signal 010. The entities of the signal generators 421 and422 are, for example, processing processes generated for everytransmission target. While the signal generator 421 ordinarily operatesfor transmitting data to the mobile station 208 within the cell 12, thesignal generator 422 starts to operate when the mobile station 209 inthe neighboring cell 13 is located in the vicinity of the cell boundary(described below). When the mobile stations 208 and 209 perform themulti-user detection based on the strength of the received signals, likethe SIC, the radio transmitter 424 preferably exchanges the transmissionpower for the user data for that for the control information to outputthem.

[0100] The base station 103 includes a signal generator 431 forgenerating a transmitted signal 039 that has user data for the mobilestation 209 in its local cell, a signal generator 432 for generating atransmitted signal 038 that has control information for the mobilestation 208 outside its local cell, a multiplexer 433 for mixing the twotransmitted signals by time division multiplexing or the like, and aradio transmitter 434 for transmitting a multiplexed signal 013 as atransmitted signal 014. The entities of the signal generators 431 and432 are, for example, processing processes generated for everytransmission target. While the signal generator 431 ordinarily operatesfor transmitting data to the mobile station 209 within the cell 13, thesignal generator 432 starts to operate when the mobile station 208 inthe neighboring cell 12 is located in the vicinity of the cell boundary(described below). When the mobile stations 208 and 209 perform themulti-user detection based on the strength of the received signals, likethe SIC, the radio transmitter 434 preferably exchanges the transmissionpower for the user data for that for the control information to outputthem.

[0101] The mobile station 208 has a multi-user detector 4008. Thetransmitted signal 010 from the base station 102 is supplied as areceived signal 011 to the mobile station 208 through a propagation pathhaving a propagation characteristic H28, and the transmitted signal 014from the base station 103 is also supplied as a received signal 015 tothe mobile station 208 through a propagation path having a propagationcharacteristic H38. The multi-user detector 4008 detects the receivedsignal 01 for identifying the user by the multi-user detection functionsuch as the SIC to reproduce a signal 028′ as the signal generated bythe signal generator 421 and to reproduce a signal 029′ as the signalgenerated by the signal generator 422. Similarly, the multi-userdetector 4008 detects the received signal 015 for identifying the userto reproduce a signal 039′ as the signal generated by the signalgenerator 431 and to reproduce a signal 038′ as the signal generated bythe signal generator 432.

[0102] The mobile station 209 has a multi-user detector 4009. Thetransmitted signal 010 from the base station 102 is supplied as areceived signal 012 to the mobile station 209 through a propagation pathhaving a propagation characteristic H29, and the transmitted signal 014from the base station 103 is also supplied as a received signal 016 tothe mobile station 209 through a propagation path having a propagationcharacteristic H39. The multi-user detector 4009 detects the receivedsignal 012 for identifying the user by the multi-user detection functionsuch as the SIC to reproduce a signal 028″ as the signal generated bythe signal generator 421 and to reproduce a signal 029″ as the signalgenerated by the signal generator 422. Similarly, the multi-userdetector 4009 detects the received signal 016 for identifying the userto reproduce a signal 039″ as the signal generated by the signalgenerator 431 and to reproduce a signal 038″ as the signal generated bythe signal generator 432.

[0103] The operating procedure for the mobile station to perform thehandoff in the multi-cell multiple-access radio communication systemstructure shown in FIGS. 5 to 7 will now be described with reference toa sequence diagram in FIG. 8.

[0104] (1) Propagation Environment on Cell Boundary between MobileStation and Base Station

[0105] For example, a case in which the mobile station 208 moves fromthe cell 12 toward the cell 13 will be considered. In this case, sincethe mobile station 208 in the cell 12 is located in the vicinity of theboundary with the cell 13, the base station 103 can also receive thesignal from the mobile station 208 located in the adjacent cell 12 inthe up-line reception, from the point of radio-wave propagation. Thebase station 103 considers this signal as a signal targeted at the basestation 103 to perform the multi-user detection.

[0106] (2) Each Mobile Station Located around Center of Its Local Cell

[0107] The mobile station 208 basically communicates with the basestation 102 in the cell 12. Specifically, the mobile station 208receives the information including the user data in the signal generator381 to generate the signal 082, which is emitted on a radio channel asthe signal 002 through the radio transmitter 384 without beingmultiplexed. Upon reception of the signal 003 through the propagationcharacteristic H82, the base station 102 demodulates the signal 003 tothe signal 082′ by the multi-user detection. Transmission and receptionoperation of the user data, similar to the above operation, is performedbetween the mobile station 209 and the base station 103.

[0108] (3) Arrival of Mobile Station in the Vicinity of Cell Boundary

[0109] When the mobile station 208 arrives in the vicinity of the cellboundary under the condition described above, the signal 002 emitted onthe radio channel reaches the base station 103 as the signal 004 throughthe propagation characteristic H83. The base station 103 provided withthe multi-user detector 3003 detects the signal 082″ resulted from themulti-user detection. Hence, the base station 103 can be aware that themobile station 208 is located in the vicinity of the cell boundary onthe same channel even during communication with the mobile station 209.

[0110] (4) Start of Mixture at Base Station

[0111] Upon awareness of the entrance of the mobile station 208 into thelocal cell, the base station 103 mixes some kind of response in theincoming mobile station 208 while being in communication with the mobilestation 209 in the local cell. The some kind of response may be messagedata including an expression, such as “on the cell boundary”or“visible”, toward the corresponding mobile station and this message datais used as the control information for the handoff operation.

[0112] As shown in FIG. 7, the base station 103 generates the signal 039for the mobile station 209 in the signal generator 431 and the signal038 for the mobile station 208 in the signal generator 432. The basestation 103 synthesizes the signals 039 and 038 in the multiplexer 433by the TDMA or the like to generate the signal 013.

[0113] An example of a packet synthesized by the TDMA is shown in FIG.12. A header section including destination information and so on, a datatransmission section for the mobile station 209, which is the originalcommunication target, in the local cell, a data transmission section forthe mobile station 208, which is not the original communication target,a received signal section, and a CRC (cyclic redundancy checksum)section for error control are multiplexed in time series. Methods fordistinguishing a signal targeted at the original destination and asignal targeted at a station other than the original destination includea method in which the header section is used to specify the leading edgeof data.

[0114] The base station 103 supplies the generated signal 013 to theradio transmitter 434 for emitting it as the radio signal 014 on theradio channel. Through the radio channel, the radio signal 014 reachesthe mobile station 209 that is located in its local cell as the signal016 through the channel H39 and reaches the mobile station 208 as thesignal 015 through the channel H38.

[0115] The mobile station 209 having the multi-user detection functiondetects the received signal 016 by the multi-user detection to reproducethe signals 039″ and 038″. Similarly, the mobile station 208 detects thereceived signal 015 by the multi-user detection to reproduce the signals039′ and 038′. Then, the mobile station 208 proceeds to the nextoperation mode when it recognizes that the incoming signal 038′ from thebase station 103 is not a signal from the base station in its localcell. As described above, the signal 038 includes an expression, such as“on the cell boundary” or “visible”, and the signal 038 is the controlinformation indicating that signals from the base station 103 in theneighboring cell 13 can be received.

[0116] (5) Response and Start of Mixture in Mobile Station

[0117] When the mobile station 208 is informed that it enters thereception area of the base station 103 from the base station 103 in theneighboring cell 13, the mobile station 208 continues to generate thecommunication information 082 for the base station 102 in its local cellin the signal generator 381, like before. This information follows theinformation that has been communicated in the cell 12 by the mobilestation 208.

[0118] The mobile station 208 generates as the handoff information theinformation 083 indicating that it requests the handoff in the signalgenerator 382 and synthesizes the signals 082 and 083 in the multiplexer383 by the TDMA or the like. The synthesized signal 001 is converted tothe transmitted signal 002 in the radio transmitter 384 to be emitted onthe radio channel.

[0119] On the other hand, the transmitted signal 002 reaches the basestation 102 as the signal 003 through the propagation characteristic H82and reaches the base station 103 as the signal 004 through thepropagation characteristic H83. At the base station 102, the signal 003continues to be demodulated to the signal 082′ to be supplied from themobile station 208 to the base station 102 by the multi-user detection.The base station 103 confirms that the mobile station 208 hasacknowledged the reception of the handoff, by demodulating and detectingthe handoff-enabling signal 083″ for the base station 103 from thesignal detected by the multi-user detection.

[0120] (6) Mixed Communication at Both Mobile Station and Base Station

[0121] The base station 103 generates the information 038 including thecontrol information and so on in the signal generator 432 and, at thesame time, generates the communication information 039 for the mobilestation 209 in its local cell 13 in the signal generator 431. The basestation 103 synthesizes the signals 038 and 039 in the multiplexer 433.The synthesized signal 013 is converted into the transmitted signal 014in the radio transmitter 434 to be emitted on the radio channel.

[0122] The transmitted signal 014 reaches the mobile station 209 as thesignal 016 through the radio channel H39 and reaches the mobile station208 as the signal 015 through the radio channel H38. The mobile station209 detects the signal 039″ that is supplied from the base station 103in its local cell in the multi-user detector 4009 by the multi-userdetection and then demodulates the detected signal. The mobile station208 that has requested the handoff can demodulate and detect theinformation 038′, such as the control information for the mobile station208, by performing the multi-user detection in the multi-user detector4008, as in the same manner, to receive the control information, thedata information, and so on from the base station 103 in the neighboringcell 13.

[0123] After the handover is determined, the mobile station 208transmits a disconnection request to the base station 102 and starts tocommunicate only with the base station 103. In other words, in themobile station 208, the operation of the signal generator 381 issuspended and only the signal generator 382 operates for continuing totransmit the transmitted signal 083 to the base station 103.

[0124] In a series of handoff process described above, the handoff isachieved between the base stations 102 and 103 only with the handoffrequest from the mobile station 102 without any connection therebetween.Specifically, a base station can receive by the multi-user detection thecontrol information or data including the availability of the handover,which has not been transmitted to the base station, among theinformation concerning a terminal located outside its local cell. Inother words, the amount of traffic on a backbone does not need to beincreased in the handover of a mobile station. The multi-user detectionis characterized by ordinarily occurring in the vicinity of the cellboundary, from the point of propagation characteristic, so that themulti-user detection probably has an affinity to the handover.Furthermore, the start of the mixture of the control signal for thefirst time upon detection of the mobile station that approaches the cellboundary permits the effective utilization of the bandwidth.

[0125]FIG. 9 is a flowchart showing the operating procedure of a radiocommunication apparatus operating as a mobile station in the multi-cellradio communication environment according to this embodiment.

[0126] The mobile station has the multi-user detection function. Themobile station tries to demodulate both the signals for itself withinits local cell and the signals for other stations,. supplied fromoutside its local cell, (Step S1), and determines whether the signalsfrom outside its local cell exist (Step S2).

[0127] If the signals from outside the local cell do not exist in thereceived signals, the mobile station transmits only the signals for thebase station in its local cell (Step S3).

[0128] If the signals from outside the local cell are detected by themulti-user detection, it is determined whether the handoff is performed(Step S4). Since the detection of the signals from outside the localcell would mean that the mobile station approaches the boundary with theneighboring cell and that the field intensity of the received signalsfrom the base station in connection decreases to lower the communicationquality, the availability of the handoff is determined in considerationof such situations.

[0129] If it is determined that the handoff is not performed, only thesignals for the base station in its current local cell are transmitted(Step S5). If it is determined that the handoff is performed, themixture of the signals for the base station in its current local celland the signals for the base station of the neighboring cell istransmitted (Step S6). For example, the information, such as the handoffrequest for the base station of the neighboring cell or the reception ofthe handoff request, is mixed to be transmitted to the base station ofthe neighboring cell.

[0130]FIG. 10 is a flowchart showing the operating procedure of a radiocommunication apparatus operating as a base station in the multi-cellradio communication environment according to this embodiment.

[0131] The base station has the multi-user detection function. The basestation tries to demodulate both the signals for itself within its localcell and the signals for other stations, supplied from outside its localcell, (Step S11) and determines whether the signals from outside itslocal cell exist. (Step S12).

[0132] If the signals from outside the local cell do not exist in thereceived signals, the base station transmits only the signals for themobile station in its local cell (Step S13).

[0133] If the signals from outside the local cell are detected by themulti-user detection, it is determined whether the handoff is enabled(Step S14). Since the detection of the signals from outside the localcell would mean that the mobile station in the neighboring cellapproaches the boundary with the local cell, the availability of thehandoff is determined in consideration of such situations.

[0134] If it is determined that the handoff is not performed, only thesignals for the mobile station in its current local cell are transmitted(Step S15). If it is determined that the handoff is enabled, the mixtureof the signals for the mobile station in its current local cell and thehandoff-request enabling signal for the mobile station of theneighboring cell is transmitted (Step S16).

[0135] The operation sequence of the handoff shown in FIG. 8 is involvedin the base-station-driven handover based on the multi-user detection ofthe signals from the mobile station 208 located in the vicinity of theneighboring cell by the base station 103. In contrast,mobile-station-driven handover can be performed based on the multi-userdetection of the signals from the base station of the neighboring cellby the mobile station. The latter handoff procedure will now bedescribed with reference to the operation sequence shown in FIG. 11.

[0136] (1) Propagation Environment on Cell Boundary between MobileStation and Base Station

[0137] For example, a case in which the mobile station 208 moves fromthe cell 12 toward the cell 13 will be considered. In this case, sincethe mobile station 208 in the cell 12 is located in the vicinity of theboundary with the cell 13, the mobile station 208 can also receive thesignal from the base station 103 in the adjacent cell 13 in thedown-line reception, from the point of radio-wave propagation. Themobile station 208 considers this signal as a signal targeted at themobile station 208 to perform the multi-user detection.

[0138] (2) Each Mobile Station Located around Center of Its Local Cell

[0139] The mobile station 208 basically communicates with the basestation 102 in the cell 12. Specifically, the base station 102 receivesthe information including the user data in the signal generator 421 togenerate the signal 028, which is emitted on a radio channel as thesignal 010 through the radio transmitter 424 without being multiplexed.Upon reception of the signal 011 through the propagation characteristicH28, the mobile station 208 demodulates the signal 011 to the signal028′ by the multi-user detection. Transmission and reception operationof the user data, similar to the above operation, is performed betweenthe base station 103 and the mobile station 209. Namely, the basestation 103 receives the information including the user data in thesignal generator 431 to generate the signal 039, which is emitted on theradio channel as the signal 014 through the radio transmitter 434without being multiplexed.

[0140] (3) Arrival of Mobile Station in the Vicinity of Cell Boundary

[0141] When the mobile station 208 arrives in the vicinity of the cellboundary under the condition described above, the signal 014 emitted onthe radio channel by the base station 209 reaches the mobile station 103as the signal 015 through the propagation characteristic H38. The mobilestation 208 provided with the multi-user detector 4008 detects thesignal 039″ resulted from the multi-user detection. Hence, the mobilestation 208 can be aware that the mobile station 208 is in the vicinityof the cell boundary toward the base station 103 on the same channeleven during communication with the base station 102.

[0142] (4) Start of Mixture at Mobile Station

[0143] Upon awareness of being in the vicinity of the boundary with theneighboring cell 13, the mobile station 208 sends some kind of responseto the base station 103 in the neighboring cell 13 while being incommunication with the base station 102 in the cell 12 where the mobilestation 208 is located. The some kind of response may be message dataincluding an expression, such as “on the cell boundary” or “visible”,and this message data is used as the control information for the handoffoperation.

[0144] As shown in FIG. 6, the mobile station 208 generates the signal082 for the base station 102 in the signal generator 381 and the signal083 for the base station 103 in the signal generator 382. The mobilestation 208 synthesizes the signals 082 and 083 in the multiplexer 383by the TDMA or the like to generate the signal 001. The mobile station208 supplies the generated signal 001 to the radio transmitter 384 foremitting it as the radio signal 002 on the radio channel. Through theradio channel, the radio signal 002 reaches the base station that iscurrently in connection with as the signal 003 through the channel H82and reaches the base station 103 in the neighboring cell 12 as thesignal 004 through the channel H83.

[0145] The base station 103 having the multi-user detection functiondetects the received signal 004 by the multi-user detection to reproducethe signals 082″ and 083″. Similarly, the base station 102 detects thereceived signal 003 by the multi-user detection to reproduce the signals082′ and 083′. Then, the base station 103 proceeds to the next operationmode. when it recognizes that the incoming signal 083″ from the mobilestation 208 is not a signal from a mobile station in the local cell 13.As described above, the signal 083 includes an expression, such as “onthe cell boundary” or “visible”, and the signal 083 is the controlinformation indicating that signals from the mobile station 208 in thevicinity of the boundary with the neighboring cell 12 can be received.

[0146] (5) Response and Start of Mixture in Base Station

[0147] When the base station 103 is informed that the mobile station 208in the vicinity of the boundary with the neighboring cell 12 enters thereception area of the base station 103, the base station 103 continuesto generate the communication information 039 for the mobile station 209in the local cell 13 in the signal generator 431, like before. Thisinformation follows the information that has been communicated in thecell 13 by the base station 103.

[0148] The base station 103 generates as the handoff information thehandoff-enabling signal 038 in response to the handoff request in thesignal generator 432 and synthesizes the signals 039 and 038 in themultiplexer 433 by the TDMA or the like. The synthesized signal 013 isconverted to the transmitted signal 014 in the radio transmitter 434 tobe emitted on the radio channel.

[0149] On the other hand, the transmitted signal 014 reaches the mobilestation 208 as the signal 015 through the propagation characteristic H38and reaches the mobile station 209 as the signal 016 through thepropagation characteristic H39. At the mobile station 209, the signal016 continues to be demodulated to the signal 039″ to be supplied fromthe base station 103 to the mobile station 209 by the multi-userdetection. The mobile station 208 confirms that the base station 103 hasacknowledged the reception of the handoff, by demodulating and detectingthe handoff-enabling signal 038′ for the mobile station 208 from thesignal detected by the multi-user detection.

[0150] (6) Mixed Communication at Both Mobile Station and Base Station

[0151] The mobile station 208 generates the information 083 includingthe control information and so on in the signal generator 382 and, atthe same time, generates the communication information 082 for the basestation 102 in its local cell 12 in the signal generator 381. The mobilestation 208 synthesizes the signals 082 and 083 in the multiplexer 383.The synthesized signal 001 is converted into the transmitted signal 002in the radio transmitter 384 to be emitted on the radio channel.

[0152] The transmitted signal 002 reaches the base station 102 as thesignal 003 through the radio channel H82 and reaches the base station103 as the signal 004 through the radio channel H83. The base station102 detects the signal 082′ that is supplied from the mobile station 208in its local cell 12 in the multi-user detector 3002 by the multi-userdetection and then demodulates the detected signal. The base station 103that has submitted the handoff-enabling signal can demodulate and detectthe information 083″, such as the control information for the basestation 103, by performing the multi-user detection in the multi-userdetector 3003, as in the same manner, to receive the controlinformation, the data information, and so on from the mobile station 208in the neighboring cell 12.

[0153] After the handover is determined, the mobile station 208transmits a disconnection request to the base station 102 and starts tocommunicate only with the base station 103. In other words, in themobile station 208, the operation of the signal generator 381 issuspended and only the signal generator 382 operates for continuing totransmit the transmitted signal 083 to the base station 103.

[0154] In a series of handoff process described above, the handoff isachieved between the base stations 102 and 103 only with the handoffrequest from the mobile station 102 without any connection therebetween.Specifically, a base station can receive by the multi-user detection thecontrol information or data including the availability of the handover,which has not been transmitted to the base station, among theinformation concerning a terminal located outside its local cell. Inother words, the amount of traffic on the backbone does not need to beincreased in the handover of a mobile station.

[0155] Methods for starting the terminal-driven handover request (thatis, the handover request without the backbone), as shown in FIGS. 8 and11, include a method in which the field strength is monitored todetermine that a mobile station is located in the vicinity of the cellboundary if the received field strength from a desired base stationdecreases and a method in which it is determined whether a mobilestation is located in the vicinity of the cell boundary by usingposition information acquired by a GPS (Global Positioning System) orthe like or by using a method for measuring the distance between themobile station and the base station with radio waves. Such methods maybe automatically performed by a system or may be manually performed bythe user.

[0156] According to the embodiment described above, the mixture of thesignals for a station outside the local cell is not started until it isdetermined that the mobile station is located in the vicinity of thecell boundary. In such a case, broader bandwidth can be allocated forthe transmission of user data without the handover, thus improving thedata transmission efficiency. Although various criteria for starting themixture of the signals can be offered, they-are not mentioned in thisdescription.

[0157] In this description, “being within a cell” means to be within thecell of the base station to which a mobile station belongs (on which amobile station is registered). The multi-user detection is performed notonly when the mobile station is located in the vicinity of the cellboundary but also when the mobile station is located around the centerof its local cell. In the latter case, the interference from otherstations is decreased. It is possible to categorize signals into“signals for the local station” and “other signals”. When a mobilestation is located in the boundary between two cells and the handoff isto be performed, both the signals for the mobile station from the twobase stations and other signals are received as desired signals.

[0158]FIG. 13 is a diagram showing an example in which the presentinvention is applied to the handoff in a hot-spot service.

[0159] In recent years, hotspot services that offer an Internet-accessservice by installing access points for a wireless LAN in a store suchas a coffee shop have become widespread. Under the present situation,each access point is only connected to the Internet, so that the usercannot move between hotspots while enabling the handoff. This is becausethe access points are planlessly provided by the respective stores and amechanism for controlling a plurality of access points (a mobilecommunication control center (MSC) for a cellular system) does notexist. An “access point” here is substantially synonymous with a basestation.

[0160] Conducting the respective handoff processes according to thepresent invention shown in FIGS. 8 and 11 by access points and mobileterminals, each having the multi-user detection, can realize the handoffwithout a mechanism for controlling the plural access points, as shownin FIG. 13.

[0161] Each access point, such as a drug store or a coffee shop, is onlyconnected to the Internet; whereas each mobile station seamlesslyrealizes the handoff while establishing a connection with each of theaccess points. There is no need for providing the plural access pointsaccording to the plan.

[0162]FIG. 14 is a diagram showing an example in which the presentinvention is applied to the handoff in a ubiquitous network.

[0163] In recent years, ubiquitous networks in which all electricalequipment in a house or an office is connected over a network have beenof interest to people. In a ubiquitous network, different access pointsare provided in a living room and a private room, as shown in FIG. 14.

[0164] Although a mechanism for controlling a plurality of access pointsis required in a known handoff process, conducting the handoff processaccording to the present invention shown in FIGS. 8 and 11 can realizethe handoff without the mechanism for controlling a plurality of accesspoints.

[0165] In this manner, it is possible for the mobile station toseamlessly realize the handoff while establishing a connection with eachaccess point even when the user randomly provides electrical equipmentthat is an access point.

[0166] Supplement

[0167] The present invention has been described in its preferred formwith a certain degree of particularity. However, it will be obvious tothose skilled in the art that various changes and modifications may bemade in the invention without departing from the spirit and scopethereof. The present invention is therefore illustrative and notrestrictive. It is to be understood that the scope of the invention isdetermined by the following claims.

[0168] Industrial Applicability

[0169] The present invention can provide a superior radio communicationsystem in which a plurality of mobile terminals simultaneouslycommunicates with one base station by multiple-access, a radiocommunication apparatus and radio communication method, and a computerprogram.

[0170] The present invention can provide a superior radio communicationsystem in which a mobile station can switch the base station to anotherbase station outside its local cell without requiring connection betweenthe base stations, for example, during handoff or when the cell isoverloaded, a radio communication apparatus and radio communicationmethod, and a computer program.

[0171] The present invention can provide a superior radio communicationsystem in which the mobile-terminal-driven handoff can be realized evenif the base stations are planlessly provided, a radio communicationapparatus and radio communication method, and a computer program.

[0172] According to the present invention, handoff start signals andhandoff stop signals are unnecessary as independent signals, thusimproving the throughput of the entire communication system.

1. A radio communication system having a multi-cell multi-user structure in which cells where the respective base stations are capable of communicating are adjacently or closely arranged, wherein each base station that has a multi-user detection function determines that a mobile station outside its local cell approaches the boundary with a neighboring cell based on the detection of a transmitted signal from the mobile station for identifying a user, and mixes and synthesizes a signal for a mobile station within the local cell and a signal for the mobile station outside the local cell to output the synthesized signal.
 2. A radio communication system according to claim 1, wherein the mobile station outside the local-cell transmits a request for handoff to the base station in the neighboring cell and disconnects from an original base station for the handoff in response to the handoff that is enabled.
 3. A radio communication system having a multi-cell multi-user structure in which cells where the respective base stations are capable of communicating are adjacently or closely arranged, wherein a mobile station that has a multi-user detection function determines that the mobile station approaches the boundary with a neighboring cell based on the detection of a transmitted signal from a base station outside its local cell for identifying a user, and mixes and synthesizes a signal for the base station within the local cell and a signal for the base station in the neighboring cell to output the synthesized signal.
 4. A radio communication system according to claim 1, wherein the base station in the neighboring cell transmits a handoff-enabling signal in response to the reception of a signal from the mobile station, and wherein the mobile station disconnects from an original base station for handoff in response to the handoff that is enabled.
 5. A radio communication apparatus operating in a radio communication environment that has a multi-cell multi-user structure in which cells where the respective base stations are capable of communicating are adjacently or closely arranged, the radio communication apparatus comprising: multi-user detection means for detecting a plurality of received signals as desired signals; and multiple-signal transmission means for mixing and synthesizing a signal for a station within a local cell and a signal for a station outside the local cell in response to the detection of a transmitted signal from the station outside the local cell for identifying a user by the multi-user detection means to output the synthesized signal.
 6. A radio communication apparatus according to claim 5, wherein the multi-user detection means detects all the received signals by demodulating the received signals each being the sum of noise and an incoming signal that is transmitted from each transmitting station within and outside the local cell and is propagated through each propagation characteristic, in the descending order of its received power, and repeating a process of canceling the signals at a receiving station.
 7. A radio communication apparatus according to claim 6, wherein the multiple-signal transmission means exchanges the transmission power of a signal for a station within the local cell for that of a signal for a station outside the local cell to output them.
 8. A radio communication apparatus according to claim 5, wherein the radio communication apparatus operates as a base station that is connected to one or more mobile stations in a cell where the base station is capable of communicating, wherein the multiple-signal transmission means mixes and synthesizes a signal for a mobile station within the local cell and a signal for a mobile station outside the local cell to output the synthesized signal, upon determining that the mobile station outside the local cell approaches the boundary with a neighboring cell based on the detection of a transmitted signal from the mobile station outside the local cell for identifying the user by the multi-user detection means, and wherein the radio communication apparatus enables handoff in response to a request for the handoff from the mobile station outside the local cell.
 9. A radio communication apparatus according to claim 5, wherein the radio communication apparatus operates as a mobile station that is located in a cell provided by a base station, wherein the multiple-signal transmission means mixes and synthesizes a signal for the base station within the local cell and a signal for the base station in a neighboring cell to output the synthesized signal, upon determining that the mobile station approaches the boundary with the neighboring cell based on the detection of a transmitted signal from a base station outside the local cell for identifying the user by the multi-user detection means, and wherein the radio communication apparatus performs handoff in response to handoff allowance from the base station in the neighboring cell.
 10. A radio communication method in a radio communication environment that has a multi-cell multi-user structure in which cells where the respective base stations are capable of communicating are adjacently or closely arranged, the radio communication method comprising: a multi-user detection step for detecting a plurality of received signals as desired signals; and a multiple-signal transmission step for mixing and synthesizing a signal for a station within a local cell and a signal for a station outside the local cell to output the synthesized signal in response to the detection of a transmitted signal from the station outside the local cell for identifying a user in the multi-user detection step.
 11. A radio communication method according to claim 10, wherein the multi-user detection step detects all the received signals by demodulating the received signals each being the sum of noise and an incoming signal that is transmitted from each transmitting station within and outside the local cell and is propagated through each propagation characteristic, in the descending order of its received power, and repeating a process of canceling the signals at a receiving station.
 12. A radio communication method according to claim 10, wherein the multiple-signal transmission step exchanges the transmission power of a signal for a station within the local cell for that of a signal for a station outside the local cell to output them.
 13. A radio communication method according to claim 10, for the operation as a base station that is connected to one or more mobile stations in a cell where the base station is capable of communicating, wherein the multiple-signal transmission step mixes and synthesizes a signal for a mobile station within the local cell and a signal for a mobile station outside the local cell to output the synthesized signal, upon determining that the mobile station outside the local cell approaches the boundary with a neighboring cell based on the detection of a transmitted signal from the mobile station outside the local cell for identifying the user in the multi-user detection step, and wherein the radio communication method includes a handoff-enabling step of enabling handoff in response to a request for the handoff from the mobile station outside the local cell.
 14. A radio communication method according to claim 10, for the operation as a mobile station that is located in a cell provided by a base station, wherein the multiple-signal transmission step mixes and synthesizes a signal for the base station within the local cell and a signal for the base station in a neighboring cell to output the synthesized signal, upon determining that the mobile station approaches the boundary with the neighboring cell based on the detection of a transmitted signal from a base station outside the local cell for identifying a user in the multi-user detection step, and wherein the remote communication method includes a handoff step for performing handoff in response to handoff allowance from the base station in the neighboring cell.
 15. A radio communication apparatus operating in a radio communication environment having a multi-user structure, the radio communication apparatus comprising: signal generation means for generating a plurality of transmitted signals for a plurality of transmission targets; and multiplex-synthesis transmission means for mixing and synthesizing the transmitted signals and changing the transmission power of the transmitted signals for every transmission target for output.
 16. A radio communication method in a radio communication environment having a multi-user structure, the radio communication method comprising: a signal generation step for generating a plurality of transmitted signals for a plurality of transmission targets; and a multiplex-synthesis transmission step for mixing and synthesizing the transmitted signals and changing the transmission power of the transmitted signals for every transmission target for output.
 17. A computer program that is described in a computer-readable form to cause a computer system to execute a process for a radio communication in a radio communication environment having a multi-cell multi-user structure in which cells where the respective base stations are capable of communicating are adjacently or closely arranged, the computer program comprising: a multi-user detection step for detecting a plurality of received signals as desired signals; and a multiple-signal transmission step for mixing and synthesizing a signal for a station within a local cell and a signal for a station outside the local cell to output the synthesized signal in response to the detection of a transmitted signal from the station outside the local cell for identifying a user in the multi-user detection step. 